The present invention relates to the treatment of anxiety and related disorders.
The fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV.TM.), published in 1994 by the American Psychiatric Association, Washington, D.C., defines anxiety and related disorders. These are panic disorder without agoraphobia, panic disorder with agoraphobia, agoraphobia without history of panic disorder, specific phobia, social phobia, obsessive-compulsive disorder, post-traumatic stress disorder, acute stress disorder, generalized anxiety disorder, anxiety disorder due to a general medical condition, substance-induced anxiety disorder and anxiety disorder not otherwise specified.
Anxiety disorders are generally treated by counseling or with drugs. Classes of drugs which are widely prescribed for the treatment of anxiety disorders include the benzodiazepines (such as diazepam) and buspirone hydrochloride.
The benzodiazepines were introduced during the 1960's. They have achieved widespread acceptance, but their use is nevertheless restricted due to their adverse side-effect profile, in particular their tendency to induce dependence.
Buspirone hydrochloride was introduced during the early 1980's. It lacks the dependence-inducing side effects of the benzodiazepines, but has a slow onset of action (about 4 weeks).
There is therefore a need for new drugs for the treatment of anxiety and related disorders.
Several animal models have been developed which are recognized in the art as being predictive of anxiolytic activity. These include the fear potentiated startle model, described by Davis in Psychopharmacology 62:1; 1979, Behav. Neurosci. 100:814;1986 and TiPS, January 1992 Vol. 13! 35-41, the elevated plus model described by Lister in Psychopharmacol. 92:180-185;1987, and the well-known punished--responding (conflict) model, described, for example, in "Psychopharmacology of Anxiolytics and Antidepressants", edited by S. E. File, pp. 131-153, Raven Press, New York, 1991.
In the mammalian central nervous system (CNS), the transmission of nerve impulses is controlled by the interaction between a neurotransmitter, that is released by a sending neuron, and a surface receptor on a receiving neuron, causing excitation of this receiving neuron. The benzodiazepines and buspirone hydrochloride are both believed to exert their anxiolytic effect through binding to such receptors. In particular, the benzodiazepines are believed to act by binding to GABA receptors, while buspirone hydrochloride is believed to bind to 5HT.sub.1a receptors.
L-Glutamate, which is the most abundant neurotransmitter in the CNS, mediates the major excitatory pathway in mammals, and is referred to as an excitatory amino acid (EAA). The receptors that respond to glutamate are called excitatory amino acid receptors (EAA receptors). See Watkins & Evans, Ann. Rev. Pharmacol. Toxicol., 21, 165 (1981); Monaghan, Bridges, and Cotman, Ann. Rev. Pharmacol. Toxicol., 29, 365 (1989); Watkins, Krogsgaard-Larsen, and Honore, Trans. Pharm. Sci., 11, 25 (1990). The excitatory amino acids are of great physiological importance, playing a role in a variety of physiological processes, such as long-term potentiation (learning and memory), the development of synaptic plasticity, motor control, respiration, cardiovascular regulation, emotional states and sensory perception.
Excitatory amino acid receptors are classified into two general types. Receptors that are directly coupled to the opening of cation channels in the cell membrane of the neurons are termed "ionotropic." This type of receptor has been subdivided into at least three subtypes, which are defined by the depolarizing actions of the selective agonists N-methyl-D-aspartate (NMDA), .alpha.-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA). The second general type of receptor is the G-protein or second messenger-linked "metabotropic" excitatory amino acid receptor. This second type is coupled to multiple second messenger systems that lead to enhanced phosphoinositide hydrolysis, activation of phospholipase D, increases or decreases in cAMP formation, and changes in ion channel function. Schoepp and Conn, Trends in Pharmacol. Sci., 14, 13 (1993). Both types of receptors appear not only to mediate normal synaptic transmission along excitatory pathways, but also participate in the modification of synaptic connections during development and throughout life. Schoepp, Bockaert, and Sladeczek, Trends in Pharmacol. Sci., 11, 508 (1990); McDonald and Johnson, Brain Research Reviews, 15, 41 (1990).
The metabotropic glutamate receptors are a highly heterogeneous family of glutamate receptors that are linked to multiple second-messenger pathways. Generally, these receptors function to modulate the presynaptic release of glutamate, and the postsynaptic sensitivity of the neuronal cell to glutamate excitation. The metabotropic glutamate receptors (mGluR) have been pharmacologically divided into two subtypes. One group of receptors is positively coupled to phospholipase C, which causes hydrolysis of cellular phosphoinositides (PI). This first group are termed PI-linked metabotropic glutamate receptors. The second group of receptors is negatively coupled to adenyl cyclase, which prevents the forskolin-stimulated accumulation of cyclic adenosine monophosphate (cAMP). Schoepp and Conn, Trends Pharmacol. Sci., 14, 13 (1993). Receptors within this second group are termed cAMP-linked metabotropic glutamate receptors.
It has now been found that a compound which is an agonist that acts selectively at negatively coupled cAMP-linked metabotropic glutamate receptors is effective in the fear potentiated startle and elevated plus maze models of anxiety (but does not have benzodiazepine-like efficacy in the punished-responding (conflict) model).